ECE503 - Biomedical Instrumentation Bioelectric Measurements

Assignment Task

Bioelectricity

Naturally occurring phenomena (generation of voltage/potentials from the body) arising from the flow of ionic current (ionic conduction) in the body

Ionic Conduction

• Involves the migration of ions (positive and negatively charged) throughout a region
• Model the human body as an electrolytic solution (solution with ionic concentrations)
• Potential differences (voltages) occur when concentrations of ions are different at different points
• Trillions of cells in the body (not all are electrically active)
• Able to detect the sum of the voltages produced by many cells in the body

Resting cell membrane voltage

• Assume saline (salt water) bath represents body fluid → set as the reference point (to measure voltage across the cell membrane)
• Microelectrode measures voltage in reference to ECF (-70 mV in this case)
• What forces/gradients contribute to this voltage difference?
• If all cells have a resting membrane voltage, how do signals propagate from one location to another in the body?

Biopotential electrodes

• When you attach electrodes to the skin, there is some leakage resistance and capacitance associated with the electrode interface
• This affects the instruments/circuits that are attached to the electrode and can alter the frequency response of the instrument
• Let’s see what the equivalent circuit is for an electrode-skin interface

ECG Amplifier

• External 60 Hz noise can be reduced by also providing a phase-shifted (inverted) noise signal back to the patient (sums with noise signal to zero)

• Achieved with right leg drive circuitry Tap into the differential amp (to get common mode voltage) and apply to an inverting amplifier

• Only 10 µA is injected into the body (not much!) which follows medical instrumentation standards

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